Method of regenerating an air dryer in a vehicle air system

ABSTRACT

A vehicle air system has an air dryer which is periodically regenerated by reverse flow of dry air. The regeneration threshold is variable, and determined according to the operating state of the vehicle. The operating state is in turn determined from easily measurable vehicle status parameters, preferably in the form of on/off or in/out signals.

This invention relates to an air system of a vehicle, and particularlyto active management of regeneration of the usual air dryer.

Air systems are typically used on heavy trucks and rely on air underpressure to operate the vehicle brake actuators. Air is usually suppliedfrom an engine driven compressor to a high pressure reservoir, andthence supplied to the brake actuators via a foot operated demand valve.Means are provided to bring the compressor on and off-load according toair demand; the compressor is typically taken off-load by holding adelivery valve open and/or by opening the delivery port to atmosphere.

When under load, air from the compressor is always passed through an airdryer upstream of the reservoir since moisture in the braking system mayadversely affect the life and performance of the components of thebraking system. A typical air dryer comprises a cartridge of desiccantmaterial. Periodically the air dryer must be regenerated by drivingaccumulated moisture in the desiccant out to atmosphere, and this isdone by expanding a small volume of high pressure dry air through thedesiccant in reverse; during this operation the compressor is of courseoff-load.

In normal use, pressure in the reservoir is depleted by consumercircuits, and at a predetermined lower pressure the compressor isbrought on-load. Pressure in the reservoir consequently rises until ahigher cut-off pressure is reached, at which time the compressor istaken off-load.

The vehicle braking system is a primary part of the vehicle air system.However air under pressure may also be used for e.g. air suspension,gearshift, servo assistance, door controls etc.

Conventionally desiccant regeneration is triggered as the compressor istaken off-load, and irrespective of the volume and quality of airpassing through the desiccant cartridge. Regeneration may be relativelyfrequent during some kinds of town driving if the compressor isrepeatedly allowed to reach cut off pressure. In these circumstances thedesiccant is regenerated when only a small volume of air has passedtherethrough to the reservoir, and thus occurs at well below thevolumetric design-capacity of the air dryer. The consequence is thatmuch dry air is wasted through unnecessary regeneration, and this inturn increases fuel consumption and decreases compressor life. Similarproblems occur in certain kinds of highway driving.

On the other hand, frequent application of the vehicle brakes in towndriving may result in incomplete regeneration because the relativelyhigh air consumption causes the compressor to be brought on-load beforea regeneration cycle can be completed. The desiccant may thusprogressively become rather saturated. In a conventional system, nomeasurement is made of the level of wetness and accordingly no recoverystrategy can be implemented to bring a wet air dryer to a dry state.

Direct measurement of air moisture content, and/or the level of moisturein the desiccant has been proposed but has not been adopted for reasonsof cost, reliability and technical deficiencies.

It has been proposed to trigger regeneration on a time basis, but thisis unsatisfactory because the time interval must be set according to themaximum duty cycle of the vehicle.

This means that for normal and low duty vehicle cycles (with low airconsumption), significant volumes of dry air will be frequently expandedon a time basis through the desiccant cartridge, which thus remainssignificantly below an optimum saturation level most of the time. In thecase of very frequent brake application, the timer will continually bere-set each time the compressor comes on-load. Furthermore a safetyfactor must be included, with the result that regeneration occurs morefrequently than is necessary in most vehicle duty cycles. What isrequired is a control system which can regenerate the desiccant at alevel of optimum saturation, thus reducing the wastage of dry air, andin turn avoiding unnecessary pumping by the compressor, and alsoreducing the risk of under purging during time of high air consumption(e.g. frequent brake applications).

A complication of regeneration control is that air cannot flow throughthe air dryer cartridge simultaneously in both directions. Consequently,regeneration must be inhibited at times when the compressor is on-loadin order to satisfy a demand for air in the braking system. A bettermeans of air dryer control is therefore required.

According to a first aspect of the invention there is provided a methodof regenerating an air dryer in a vehicle air system, the methodcomprising determining volumetric air flow through the air dryer,sensing operating states of the vehicle and in response thereto settinga threshold air volume, regeneration of said air dryer being initiatedat said threshold.

Such a system preferably relies on relatively easily measurableparameters, and generates logical outputs based on those parameters inorder to initiate a regeneration event. Typically apparatus forimplementing the method uses vehicle status indicators such as brakeon/off, clutch in/out, compressor on-load/off-load, which are related ina look-up table or control map in order to determine a vehicle operatingstate, such as overrun, dynamic braking, or vehicle stopped. Havingdetermined the vehicle operating state, the threshold air volume can bedetermined from e.g. a second look-up table.

Apparatus for implementing the method may comprise a microprocessoradapted to receive logical inputs indicative of vehicle status, a firstROM to permit vehicle operating state to be determined from a pluralityof said inputs, and a second ROM to permit a threshold air volume to bedetermined from said vehicle operating state.

Initiation of a regeneration event causes regeneration to occur at theearliest available opportunity. Regeneration may be delayed if inhibiteddue to overriding air demand for the air system.

The method preferably comprises the steps of setting or determining adefault regeneration threshold of the air dryer indicative of a firstvolume of air passing therethrough, and determining an activeregeneration threshold dependent on a vehicle operating state andindicative of a different volume of air passing therethrough.

The active regeneration threshold is preferably variable in real timedependent upon vehicle operating states, and in practice can overridesaid default regeneration threshold.

The active regeneration threshold preferably comprises one of aplurality of discrete values, a plurality of vehicle operatingparameters being related to determine the appropriate discrete value.

In a preferred embodiment, the default regeneration threshold is low,that is to say that a regeneration event will be initiated after arelatively low volume of air has passed through the air dryer. An activeregeneration threshold may permit a greater volume of air to passthrough the air dryer before a regeneration event is required, andseveral yet higher thresholds may be provided in response to differentvehicle operating states.

The present invention allows regeneration to be advanced or postponedunder certain circumstances, thus avoiding unnecessary usage of airwhich has been compressed and dried. Driving is a cyclical process andaccordingly it can be anticipated that optimum circumstances for pumpingand regeneration will occur from time to time. This invention allows theregeneration threshold to be varied intelligently rather than fixing athreshold on the basis of a worst case duty cycle. The invention alsoavoids the need for direct sensing of air humidity or moisture contentof the desiccant and does not rely on a timer. Nevertheless theinvention is adaptable to direct sensing of air humidity should asuitable sensor become available.

Information concerning air flow through the air dryer can be obtainedfrom vehicle engine speed, compressor swept volume, compressor gearratio and compressor on-load time. A compressor performance map may beused, in which values of engine speed and free air delivery (FAD) ratesare correlated as a graph.

A first vehicle operating state may be steady driving, and a secondvehicle operating state may be vehicle overrun, for example, coastingdownhill. A third vehicle operating state may be dynamic braking.

A fourth operating state may for example be a static braking event(vehicle stopped). The regeneration threshold may be variable over awide range.

The control system may inhibit regeneration of the air dryer in responseto a vehicle operating state such as dynamic braking (vehicle beingslowed) or very low system pressure. In the former case, a high degreeof air dryer saturation may be tolerated if the control system givesabsolute priority to air supply during events such as dynamic braking.The determination of whether the air dryer may be regenerated is thusunder active control according to the vehicle operating state, and maychange instantaneously with a change of operating state. In this wayregeneration can be postponed or advanced to an optimum time.

Such a system permits opportunistic regeneration according to thevehicle operating state, and thus overcomes the problem of air wastagewhich is a consequence of the prior control systems. This system canalso contribute to fuel saving by permitting regeneration at opportunetimes such as when the vehicle is on overrun. This in turn may permit areduction in compressor rating since output for regeneration can be moreevenly spread through a typical vehicle duty cycle, rather than beingdetermined on a time basis or triggered automatically as a thresholdpressure is reached.

The system may also overcome the problem of incomplete or insufficientregeneration as a consequence of prior control systems which cannotadapt the regeneration event to real time operating states of thevehicle.

Furthermore, the system allows regeneration to be largely inhibited attimes when the vehicle is static or moving slowly, thus avoiding suddenloud noises which may frighten pedestrians or animals.

The system may include a number of inputs to permit vehicle operatingstate to be more closely defined. These may be compressor/engine speed(indicative of air flow through the air dryer), road speed, clutchstatus (in/out), brake status (on/off) and engine status (loaded/overrunand running/stopped).

In a preferred embodiment, the selected inputs are analysed according toa control map so that a measured state of an input is uniquely combinedwith each measured state of each other input to determine the activeregeneration threshold.

Thus an embodiment may rely on five vehicle inputs, each of which may beassigned two states giving over 30 theoretical combinations, each ofwhich is assigned an appropriate discrete value corresponding to avehicle operating state. An embodiment may select between high and lowregeneration thresholds, one of which may correspond to the defaultregeneration threshold, so that each vehicle operating state results inselection of one or other of the thresholds.

Typically the combinations may be held in a first look-up table of aROM, to permit vehicle operating state to be ascertained, and a secondlook-up table allows vehicle operating state to be associated with adesired regeneration threshold.

Such an arrangement is advantageous since it permits in use adjustmentof the regeneration thresholds according to the vehicle operatingconditions, whilst not permitting any adjustment or tampering with thefactors which first determine the vehicle operating state itself. Thusthe first look-up table is fixed by vehicle type, but the thresholdsdetermined from the second look-up table may be adjusted in useaccording to e.g. atmospheric conditions in the country of use. Thevalues in the second look-up table can for example be adjusted accordingto readily measurable conditions, such as ambient temperature, pressureand humidity, and such values may be changed dynamically with the aid ofsuitable on-board sensors notwithstanding that compressor output isnormally 100% saturated, this feature permits account to be taken ofcountries with very high or very low air moisture content.

In a simple control system according to the invention some of the inputsmay be simple on/off signals. In the case of continuously variablestates such as engine speed, the inputs may indicate low/high.Alternatively, the system may accommodate variable inputs, in discretesteps or continuously, with for example fuzzy logic for control.

In a refinement of the invention the input may be modified. Firstly theknown Free Air Delivery (FAD) rate of the compressor may be adjustedaccording to the back pressure measured in the compressor supply duct.Thus a derating factor, typically between 0.6 and 1.0 may be applied, alarger derating occurring in cases where the back pressure is high andconsequently delivery volume is reduced. The derating factor may beobtained from a compressor performance map. A second compressorperformance map may be used in which back pressure is correlated againsta compressor performance parameter such as output gallery pressure orvolumetric delivery.

Additional derating factors which may be applied include desiccantvolumetric flow, desiccant pressure and desiccant temperature. Thesederating factors may also lie in the range 0.6 to 1.0 and may bedetermined from suitable laboratory tests.

Desiccant temperature is relatively easily measured by means of a devicesuch as a thermocouple. Typically warm air has a greater capacity formoisture than cold air, and measurement of temperature allows forderating to compensate for humid conditions.

Derating factors may be applied directly to the value of an input inorder to determine more accurately the calculated air flow through theair dryer.

Derating to ensure more frequent regeneration may also be desirable incold weather in order to avoid freezing of the moving parts of the usualregeneration valve.

Other features of the invention will be apparent from the followingdescription of a preferred embodiment.

In the preferred embodiment, the control system determines one or moreregeneration thresholds for the air dryer dependent on inputs from thevehicle systems. For example, a low threshold, giving regeneration atrelatively low moisture content, may be set at times when braking demandis low. This arrangement ensures that the air dryer is maintained in arelatively dry state, rather than being allowed to accumulate moistureup to a somewhat higher predetermined regeneration threshold. A highthreshold may conversely be set at times when braking demand is high. Inthese circumstances the air dryer will be allowed to approach a maximumtolerable moisture content so as to ensure that compressor output is notinterrupted by an intermediate regeneration cycle. A yet higherthreshold may be set at times when braking demand is very high orcontinuous, in which case moisture levels in the air dryer are asecondary consideration.

Vehicle operating states are determined from easily measurableparameters and may be used to set the regeneration threshold as follows:

Vehicle Engine Stopped

This state may be determined from an ignition circuit, an enginetachometer, or the usual engine management system.

Engine cranking resistance should be reduced to a minimum for starting,and therefore the compressor should be off-load to reduce engine drag.Remaining air in the reservoirs should be reserved for braking, and notused for regeneration of the air dryer. Accordingly the regenerationthreshold is high (high moisture content permitted) and regenerationwill not be triggered until a large volume of air has passed through theair dryer after the engine has started. This volume is calculated froman engine speed input (compressor output volume being a function ofengine speed) and a compressor on-load time given by a system clock andan on-load/off-load input signal.

Vehicle Engine Idling

This state may be determined from an engine speed tachometer, clutchin/out and brake off/on inputs.

The requirement for air for braking is low because the vehicle isstopped; thus air demand is low. Also, regeneration should be inhibitedto avoid sudden noise from stationary vehicle. Accordingly, regenerationthreshold is high.

Dynamic Braking

This state may be determined from a vehicle speed input and a brakeoff/on input.

Air demand is high and the compressor is most likely to be on-load inorder to replenish the air reservoir. Furthermore additional engine dragfrom an on-load compressor may be beneficial in slowing the vehicle.Accordingly, regeneration threshold is high, thus permitting maximumthroughput of air into the braking system, usually at no fuel costbecause the throttle is off during a braking event.

Static Braking

This state can be determined from a vehicle speed input and a foot brakeoff/on input or a handbrake off/on input.

Air demand is high, but compressor should be off-load if possible sinceto replenish the braking system will increase fuel usage. In thesecircumstances it is better to wait until an engine overrun condition,during which the compressor can be driven without using fuel.Regeneration threshold is accordingly high.

Engine Overrun

This state can be determined from a vehicle speed input and a throttleon/off input.

During overrun, the compressor is driven by the engine at no fuel cost.Thus this is the best time to put the compressor on-load. The vehiclewill be slowed slightly due to compressor working resistance which isusually advantageous. Regeneration threshold can be high.

Driving

This state can also be determined from a vehicle speed input and athrottle on/off input.

The compressor is typically off-load, having charged the reservoir. Theregeneration threshold can be low (regeneration at low moisturecontent), thus permitting regeneration at a time when the air reservoiris full and air demand is low.

Engine overrun and driving inputs may also be inhibited below apredetermined vehicle speed so as to indicate this state only duringhighway driving.

Gear Shifting

This state can be determined from a gear selection indicator, or from apressure sensor of a servo assistance device for the gearshift lever.

Again, air demand by the braking system is low, and the compressoroff-load. Regeneration threshold can be low.

Neutral Throttle

This state indicates that engine power is being used, but not for activedriving of the vehicle. For example a Power Take Off or poweredattachment might be in use and accordingly the regeneration thresholdshould be high since air is not required for the braking system. Thisstate can be determined according to an on/off PTO switch for example.

Moving Off From Rest

This state can be determined from e.g. vehicle speed and gear selection.Noisy regeneration should be avoided at low speed. Accordingly,regeneration threshold should be high.

Pump Up

Under these circumstances the vehicle driver is demanding air, forexample for the braking system, for initial pressurization of thesystem. Accordingly the requirement for air supply overrides airquality. Regeneration threshold should be very high, or regenerationshould be inhibited under these circumstances.

A system having two regeneration levels has been described, but a systemhaving more regeneration levels is possible. For example, theregeneration level during braking events (both static and dynamic) canbe very high or regeneration may be inhibited on the basis that thecompressor should be able to supply air without delay should the brakesystem have an air demand. This modification simply requires analternative threshold air volume to be stored in the ROM of thepreferred embodiment.

1. A method of controlling regeneration of an air dryer in a vehicle airsystem of a vehicle, the method comprising: providing a vehicle airsystem with an air dryer; determining volumetric air flow through theair dryer; sensing operating states of the vehicle; setting a thresholdair volume in response to the sensed operating states of the vehicle;and initiating regeneration of said air dryer at said threshold airvolume to regenerate said air dryer of said vehicle air system.
 2. Amethod according to claim 1 and further including the steps of settingor determining a default regeneration threshold of the air dryerindicative of a first volume of air passing therethrough, anddetermining an active regeneration threshold dependent on a vehicleoperating state and indicative of a different volume passingtherethrough, said threshold air volume being one of said activeregeneration threshold and default regeneration threshold.
 3. A methodaccording to claim 2 wherein said active regeneration threshold isvariable in real time, depending upon vehicle operating states.
 4. Amethod according to claim 3 wherein said active regeneration thresholdcomprises one of a plurality of discrete values, a plurality of vehicleoperating parameters being summed to determine said one of saidplurality of discrete values.
 5. A method according to claim 4 whereinsaid default regeneration threshold is lower than said activeregeneration threshold.
 6. A method according to claim 1 wherein a firstvehicle operating state is steady driving, a second vehicle operatingstate is vehicle overrun, a third vehicle operating state is dynamicbraking and a fourth operating state is a static braking event.
 7. Amethod according to claim 6 and further including receiving a pluralityof inputs to determine said vehicle operating states, said inputs beingone or more of compressor/engine speed, road speed, clutch status, brakestatus and engine status.
 8. A method according to claim 7 wherein oneor more of said inputs are on/off signals.
 9. A method according toclaim 1 and further including applying at least one of a plural deratingfactors for reducing said threshold air volume, said plural deratingfactors being applied based on predetermined values of at least one ofcompressor output back pressure, desiccant volumetric flow, desiccantpressure and desiccant temperature.
 10. A method according to claim 9wherein said derating factors lie in the range 0.6–1.0.